The spinal canal is a cylindrical space in a vertical direction located in the center of a backbone (corpus vertebrae), and a spinal cord and cauda quine (nerve) are housed therein and protected firmly. Spinal canal stenosis where this spinal canal is stenosed due to various causes such as deformation of a bone, a cartilage or a ligament to press the nerve therein has become a major social problem as the number of patients has exceeded those suffering from a herniated disc.
In particular, lumbar spinal canal stenosis refers to a state where the deformation and thickening of lumbar vertebra and intervertebral joints as well as deformation and bulging of the intervertebral disc which is cartilage tissue, or hypertrophy of a ligament occurs and these make the spinal canal narrow to apply pressure or squeeze the nerves and blood vessels.
As conservative therapeutic methods which improve symptoms in daily life, physical therapies such as drug therapy of administering a drug which is a vasodilator drug or increases blood flow to a nerve root or a periphery of the cauda quine, an epidural block method, a radicular block method, an orthosis therapy of wearing a lumbar vertebra bending position corset to keep the lumbar portion at rest at the bending position, and an ultrasonic therapy and a hot pack therapy for improving pain relief, muscular spasticity and blood circulation are available.
When the conservative therapy is ineffective and severe neurological disorder and intermittent claudication sustain, a surgical therapy is performed. As a conventional surgical therapy, neurological decompression procedures such as laminectomy and expanded fenestration have been performed.
In the laminectomy and expanded fenestration, surgical invasion is applied to the patient to restore the lumbar vertebra stenosis site, and thus it is necessary to give general anesthesia to the patient. In this procedure, the patient is likely to be exposed to the risk of hemorrhaging and serious complications, and hospitalization for several days to several weeks is required for a patient after the operation. Therefore, this procedure heavily burdens the patient, and particularly when the patient is elderly, the symptom is sometimes further worsened.
In recent years, it has been reported that by stationing an interspinous process spacer in a minor surgical method, the effect of local lumbar anteflexion is obtained and satisfactory results are obtained (Patent Document 1,2).
Such a minimally invasive procedure can be performed under a local anesthesia. Thus, a shorter period is needed for recovery, there is almost no hemorrhaging, the risk of serious complications is reduced and therapeutic cost required for the patient is less. Therefore, it has been desired that spinal canal stenosis can be treated using the minimally invasive procedure.
The present inventor has already proposed an interspinous implant for the purpose of realizing a more minimally invasive therapeutic method for spinal canal stenosis, while allowing simply stationing an interspinous spacer without the need for a large skin incision and also without the need for detaching the paraspinal muscle from the spine (Patent Document 3).
It is an unprecedented implant with an advantage such that it can be inserted obliquely instead of exactly from the side surface on the assumption that the spacer is inserted from processus spinosus side. It is therefore possible to insert the implant in the shortest linear route from the outside of the body and further minimize invasion to surrounding tissues.
The proposed interspinous implant includes a substantially conoid screw region to be screwed into a processus spinosus interspace, a spacer region formed in a longitudinal direction of the screw region, a head region which is interlockable with a tool arbitrarily or attachable to a coupling member arbitrarily, and a through-hole in an axial center of the screw region, the spacer region and the head region.
According to the proposed interspinous implant, a processus spinosus interspace can be enlarged smoothly while the screw region is screwed and inserted in the processus spinosus interspace, and the spacer region is pinched by passing the screw region through the processus spinosus interspace. Thus, eventually, the space between adjacent processus spinosus can be enlarged and fixed (a predetermined distance can be maintained) and it also becomes possible to insert and station the interspinous implant percutaneously even under local anesthesia, whereby a minimally invasive operation can be realized.
Moreover, the proposed interspinous implant far exceeds the prior and existing spacer device in that the interspinous implant can be stationed without the need for detaching the paraspinal muscle from the spine, and an early effect after operation can be anticipated with the possibility of an outpatient, making it possible to further reduce temporal, physical and financial burdens on the patient.
[Patent document1]U.S. Patent Publication No. 2005/0165398
[Patent document 2] WO 2004/105656
[Patent document 3] WO 2007/018114
In the above implant stationed in a processus spinosus interspace in lumbar vertebra, presumably a considerable stress is applied to the processus spinosus which comes in contact with the implant. Such a stress may possibly result in bone destruction of the processus spinosus.
The present invention aims at reducing a dynamic stress excessively applied to processus spinosus resulting from insertion of an interspinous implant and also providing an interspinous implant which can be expected to exhibit effects of inserting the implant for a long period of time.
In order to solve the above problems, the interspinous implant according to the present invention comprises:    1) a substantially conoid screw region having screw threads to be screwable into a processus spinosus interspace;    2) a head region of a substantially inverse truncated conoid shape formed coaxially with the screw region;    3) a spacer region formed in an axial direction between the screw region and the head region; and    4) a through-hole penetrating an axial center of the screw region, the spacer region and the head region, wherein
at least one slit or groove is formed in a long axis direction of the entire shape of the implant by occupying more than one third of the full length of the implant in the long axis direction and the slit or groove is deep enough to reach the through-hole.
Owing to at least one slit or groove formed in a long axis direction of the entire shape of the implant with a length which is more than one third of the full length of the implant in the long axis direction and a depth reaching the through-hole, the entire implant is allowed to bend and given elasticity in a direction vertical to the long axis direction. Due to the flexibility and elasticity, it becomes possible to reduce a dynamic stress excessively applied to processus spinosus resulting from insertion of the implant. Accordingly, effects of inserting the implant for a long period of time can be anticipated.
Here, the above slit or groove is preferably formed continuously through the screw region, the spacer region and the head region.
By forming the slit or groove in the screw region, it becomes possible to reduce a dynamic stress applied to processus spinosus when the implant is inserted. Also, by forming the slit or groove in the spacer region, it becomes possible to reduce a dynamic stress applied to processus spinosus resulting from the presence of the implant in the processus spinosus interspace for a long period of time. Further, by forming the slit or groove in the head region, it becomes possible to reduce a dynamic stress applied to processus spinosus when the implant is removed from the processus spinosus interspace.
Moreover, in the event of inserting the implant, owing to a configuration such that the screw region is formed into a substantially conoid shape and a joint portion of the head region to be joined to a driver at insertion is allowed to move to some extent, when the implant is inserted to some extent, the tip end region of the implant comes in contact with an opposed intervertebral joint or other tissues and proceeds therealong while changing its direction toward where there is less resistance, resulting in the implant being fitted in appropriate alignment as a whole and stabilized (referred to as three-dimensional self-alignment function of the implant hereinafter).
More specifically, it is preferable that the slit or groove is formed continuously to cover more than half of a dimension of the substantially conoid shape of the screw region in the axial direction, a dimension of the spacer region in the axial direction and more than half of a dimension of the substantially inverse truncated conoid shape of the head region in the axial direction.
The present implant is made of the substantially conoid screw region, the head region of a substantially inverse truncated conoid shape and the spacer region formed in the axial direction between the screw region and the head region. Therefore, if the slit or groove is formed continuously to cover more than half of a dimension of the substantially conoid shape of the screw region in the axial direction, a dimension of the spacer region in the axial direction and more than half of a dimension of the substantially inverse truncated conoid shape of the head region in the axial direction, the slit or groove will have a length of about half to two thirds of the full length of the implant in the axial direction.
As a result, flexibility and elasticity are improved in the entire implant with better performance of the three-dimensional self-alignment function of the implant and it becomes possible to reduce a dynamic stress excessively applied to processus spinosus resulting from insertion of the implant.
Here, the reason why the slit or groove is to have a partial length of the full length of the screw region and the head region in the axial direction is to ensure the strength of the entire implant.
In addition, even if the above slit or groove is formed only in the spacer region, it can be preferably used.
As stated above, by forming the slit or groove in the spacer region, it becomes possible to reduce a dynamic stress applied to processus spinosus resulting from the presence of the implant interspace for a long period of time. Even with the slit or groove which is formed only in the spacer region, effects of inserting the implant for a longer period of time can be anticipated.
Here, it is preferable that a width of the slit or groove falls in a range of 0.5 to 1.5 mm. The present implant is an interspinous implant for the purpose of performing a minimally invasive operation for lumbar spinal canal stenosis and the implant itself has a diameter of about 10 to 15 mm. Therefore, the width of the slit or groove is adjusted to a range of depth allowing exhibition of flexibility and elasticity of the implant itself and depth which does not impair the strength of the implant itself.
Moreover, the entire shape of the implant is preferably a spindle shape. By forming a three-dimensional shape of the entire implant into a spindle shape and giving some play of about 30 degrees to the driver joint part in the head region or allowing usage of a multi-axial driver, it is made easier to exhibit the three-dimensional self-alignment function of the implant at insertion and extraction of the implant.
Here, a cross section of the spacer region is preferably circular, elliptical, substantially triangular, substantially rectangular or polygonal with a plurality of slits or grooves arranged therein at even intervals.
By arranging a plurality of slits or grooves at even intervals, the strength of the implant itself can be retained and it is possible to reduce a risk of processus spinosus coming in contact with the edge of the slits or grooves.
For instance, if a cross section of the spacer region is circular or elliptical, two slits or grooves are arranged at 180° intervals (diagonally) to be seen from the axial direction, three slits or grooves are arranged at 120° intervals to be seen from the axial direction, and four slits or grooves are arranged at 90° intervals to be seen from the axial direction.
If a cross section of the spacer region is substantially triangular, in order to reduce a risk of processus spinosus coming in contact with the edge of slits or grooves, three slits or grooves are arranged in the vicinity of the center of three flat side surfaces. As a result, three slits or grooves are placed at even intervals.
Similarly, if a cross section of the spacer region is substantially rectangular, in order to reduce a risk of processus spinosus coming in contact with the edge of slits or grooves, four slits or grooves are arranged in the vicinity of the center of four flat side surfaces. As a result, four slits or grooves are placed at even intervals.
Explained next will be a tap for interspinous implant for use in inserting the interspinous implant according to the present invention into a processus spinosus interspace.
The tap for interspinous implant according to the present invention is provided with: 1) a tip end region which is formed into a conoid shape with a more acute angle than the conus of the screw region of the interspinous implant and has screw threads occupying one third to one fifth of the generatrix from the tip end of the conoid shape; 2) an axial center region extending in an axial direction of the tip end region, whose diameter is less than a diameter of the skirts of the tip end region and whose dimension in a longitudinal direction exceeds a length from an incision site to a processus spinosus interspace; 3) a grip region fitted in the axial center region; and 4) a through-hole linearly penetrating a space from the tip end region to the end of the grip region bypassing through the axial center region.
The tap for interspinous implant according to the present invention is used in a pretreatment to allow smooth insertion of the interspinous implant from a small skin incision. The tip end region of the tap for interspinous implant is formed into a more distinct conoid shape than the interspinous implant and screw threads are created therein by occupying one third to one fifth of the generatrix from the tip end of the conoid shape. This tap is used to create a passage in a processus spinosus interspace to some extent so that an interval of the processus spinosus is opened and enlarged slightly. By using the tap for interspinous implant according to the present invention in a pretreatment, there is an advantage that the screw region of the interspinous implant in which similar screw threads have been created can be inserted more easily. The screw threads are created only up to halfway in a range of one third to one fifth of the generatrix from the tip end of the conoid shape, so that opening and enlarging a processus spinosus interspace more than necessary is prevented.
A guide pin is passed through the through-hole which penetrates a space from the tip end region to the end of the grip region by passing through the axial center region so that the tap for interspinous implant can be inserted into the body along the guide pin, whereby the tip end region of the tap can be guided accurately into a processus spinosus interspace.
Using the tap for interspinous implant according to the present invention also has an advantage that the magnitude of resistance felt by the operator on the hand when the tap is inserted can be used as a reference to select an appropriate size of the interspinous implant for the patient.